The weight of a vehicle is often of crucial importance, especially to carriers in the trucking industry. In the trucking industry, the cost of delivering a load for commercial purposes is assessed to the customer according to the weight of the load and the distance it must travel. Knowledge of load weight is therefore necessary to ensure that the customer is assessed the full price of transporting the load. Weight information can also be used to optimize the load at or near the vehicle's maximum capacity.
Previously, vehicle operators have relied on private or government-operated stationary scales or weigh stations for load weight information. Usually, though, the stationary scale is located inconveniently far from the customer's loading dock. Thus, the vehicle operator has had to rely on the customer or shipping broker's quoted load weight or must travel, sometimes out of his or her way, to the nearest stationary scale for an accurate measurement. If the load weight quote proves erroneous at a stationary scale, the vehicle operator may have to return to the customer's loading dock to obtain full payment. The vehicle operator's inability to accurately determine the load weight at the loading dock, therefore, can result in wasted operator time, wasted vehicle travel mileage and time, and erroneous or fraudulent freight charges.
The weight of commercial cargo vehicles is also important from the perspective of public safety and highway maintenance. Overloading a commercial cargo vehicle can create a hazard by reducing the vehicle's stability and braking ability. An overloaded commercial cargo vehicle also causes significantly greater wear to public highways and to the vehicle itself. Governments therefore regulate vehicle weight by specifying a maximum legal load limit and fining vehicle operators for any overage. The load limit laws, however, have been enforced using the same stationary scales relied on by operators to determine a vehicle's loaded weight for pricing purposes. Law enforcement agencies have even been known to use the records of privately operated stationary scales in enforcing the load limit laws. Vehicle operators may therefore lack the ability to detect non-compliance before being subject to liability for overloading.
Accordingly, an on-board weighing system offers significant advantages over stationary scales. With on-board weighing systems, vehicle operators can determine vehicle weight at the loading dock or while under way to ensure accurate freight charge calculation, optimize load weight, and voluntarily comply with load limits.
Various prior on-board weighing devices are known. The devices have employed various weight sensor apparatus for sensing the weight of the vehicle's load, including load cells, strain gauges, displacement transducers on leaf or coil spring suspended vehicles, or pressure transducers on height-levelled, air spring suspended vehicles. The various weight sensor apparatus generate an electrical signal related to the load weight of the vehicle. Generally, the prior devices further comprise a cab-mounted read-out device for displaying the vehicle's load weight in response to a weight sensor signal.
Typically, the read-out devices operate similarly to a simple gauge and may have zeroing, offset, or gain screw adjustments. Read-out devices of this type are capable of calibrating to only a single load weight. To calibrate the device, the vehicle is weighed at a stationary scale to determine its weight with a given load. The screw adjustments are then adjusted until the device displays the correct weight of the given load. After calibration, the device is accurate at the given load weight. However, the calibration procedure does not guarantee that the device will read accurately for other than the given calibrated load weight. Re-calibration of the device to another load weight affects the accuracy of the device at the previously calibrated load weight. Since the devices are capable of calibrating to only a single load weight, the devices can assure accuracy at only the single load weight.
A further problem with prior on-board weighing devices is created by the practice common to the trucking industry of switching trailers of tractor/trailer combination trucks. Characteristics of the various weight sensor apparatus employed by weighing devices typically vary significantly even between apparatus of the same type. It is therefore necessary to recalibrate the read-out devices every time trailers are switched. Since accurate recalibration requires the use of a stationary scale, the advantages of having an on-board scale are lost with every trailer switch.
One prior on-board vehicle weighing system disclosed by Perini et al. in U.S. Pat. No. 4,832,141 overcomes some of the disadvantages of other prior systems. The Perini system determines a vehicle's weight from a weight related signal generated by a weight sensor apparatus. A cab mounted read-out device receives the signal and determines the vehicle weight. The read-out device includes an offset amplifier with a screw adjustment for shifting the DC voltage level of the signal. The signal is then converted by an analog-to-digital converter to a digital value. The digital value is used as an address for looking up the vehicle weight in a conversion table stored in a read only memory (ROM).
The read-out device has two adjustments for calibrating the system to a weight related signal generated by a particular weight sensor apparatus. First, a conversion table which most correctly correlates the weight related signal to vehicle weights is selected out of a plurality of conversion tables stored in the ROM using a set of switches. Second, the offset adjustment screw is tuned until the correct vehicle weight is displayed by the read-out device. When properly adjusted, the read-out device can determine vehicle weight fairly accurately through a range of vehicle weights.
The Perini weighing system has two major disadvantages. First, various weight sensor apparatus have different characteristics and produce weight related signals that correlate differently to the vehicle weight. Since their signals correlate differently to vehicle weight, each weight sensor apparatus typically requires a different conversion table which correctly correlates the signal to vehicle weights. If the correct conversion table for a weight sensor apparatus is not included in the ROM, the weighing system can not accurately determine vehicle weight from the signal generated by the weight sensor apparatus. A less accurate conversion table may be selected instead, but inaccurate and unreliable vehicle weight measurements will result. For the weighing system to be generally applicable to a large variety of weight sensor apparatus, a commensurate number of conversion tables must be stored in the ROM. However, increasing the number of tables stored in the ROM only increases the likelihood that the correct conversion table is included. It does not guarantee that the correct conversion table is included. Also, increasing the number of conversion tables increases the storage requirements of the ROM, consequently increasing the cost and complexity of the system.
The second major disadvantage with the Perini weighing system is that the calibration of the system is not easily reproducible. To accommodate trailer switching, it is desirable to be able to reset the weighing system for a trailer that was previously calibrated without having to repeat the calibration process involving the use of a stationary scale. Calibrating the Perini weighing system additionally involves tuning an adjustment screw. Since adjustment screws are difficult to accurately reset to a previously calibrated setting, the calibration of the Perini weighing system is not easily reproducible.